Piston pumps generally have internal shafts that reciprocate back and forth to draw liquid into the pump and then force the liquid out of the pump. Dual acting piston pumps of this type dispense liquid in both directions of the reciprocating shaft movement. Examples of dual acting pumps are disclosed in U.S. Pat. Nos. 3,160,105; 3,995,966; and 5,067,882. Generally, these pumps include a pump body formed with a longitudinally extending passageway defining a pumping chamber divided into first and second pumping sections. The pumping chamber receives a portion of the pump shaft for reciprocating movement. The first section of the pumping chamber communicates with a discharge outlet formed in the pump body and the second section of the pumping chamber communicates with an inlet that receives a source of the liquid. The pump shaft carries a check valve and, in one direction of shaft movement, the check valve is moved to a closed position and material is forced out of the first section of the pumping chamber through the discharge outlet. At the same time, additional liquid is drawn in from the liquid source through the inlet into the second section of the pumping chamber. Reverse movement of the pump shaft into the second section of the pumping chamber opens the check valve associated with the shaft and forces liquid from the second section of the pumping chamber to the first section. This moves a corresponding amount of liquid through the discharge outlet.
The SP Series and Series 3000 pumps of Nordson Corporation, Westlake, Ohio, include pump shafts with attached check valves as generally described above and further include a check valve at the pump inlet. Each of these check valves comprise free-floating solid balls. The ball at the inlet closes the inlet while the pump shaft moves from the first section of the pumping chamber to the second section as described above. The ball carried by the pump shaft alternately moves against and away from a valve seat as the pump shaft reciprocates to selectively prevent and allow liquid flow from the second section to the first section.
Although existing piston pumps perform well in many applications, certain areas are still in need of improvement. One of these areas relates to the reduction in liquid output, measurable as flow rate and pressure, that occurs as the reciprocating pump shaft changes direction. In this regard, if liquid flows back toward the pump inlet as the shaft changes direction, this reduces flow rate and pressure at the outlet. These characteristics of typical dual acting piston pumps reduce liquid output from both the pump and any downstream dispensing device as the shaft changes direction. In hot melt adhesive dispensing operations, for example, many applications require uniform liquid discharge for purposes of obtaining an adequate adhesive bond. Reduced adhesive output from a pump can reduce adhesive bead widths or dot sizes to an extent that compromises bonding strength. Some applications further require the adhesive to be discharged laterally across a gap before reaching the substrate. In these applications, a reduced flow rate or pressure can also prevent the adhesive from hitting the substrate at the correct location or from hitting the substrate altogether.
To address various problems such as those mentioned above, it would be desirable to provide a dual acting pump that minimizes irregular liquid discharge due to the change in direction of the pump shaft and, more specifically, due to back flow of liquid in the pump.